Generally, a lead-acid cell comprises a negative plate and an positive plate which are opposed to each other with a separator interposed therebetween which separator retains an electrolyte. The negative and positive plates each comprises a layer of corresponding active material and a current collector disposed in contact with said layer.
As for the current collector, conventionally, a net-like body made of lead or lead alloy has been used. To obtain such a net-like body, a casting method, a method of punching flat plates or an expand working method such as the one shown in U.S. Pat. No. 3,867,200 has been applied. To form an electrode plate using a current collector formed of such a net-like body, it has been practiced to apply to both surfaces of this current collector an active material paste obtained by mixing a lead powder, a lead oxide powder or a lead sulfate powder with water or sulfuric acid and kneading it, and dry the same.
The net-like metal body serving as the current collector is a conductor for taking out the electromotive force produced by a chemical reaction taking place in the cell and plays the role of a support for the active material, being essential for making up the electrode plate. Thus, in order for the current collector to play such a role, it has been believed necessary for the current collector to have the complicated shape of being net-like, which accounts for the fact that heretofore, in order to obtain current collectors, casting methods, punching methods and expand working methods have inevitably been employed.
However, these methods to obtain current collectors and net-like current collectors obtained thereby offer some problems to be solved. The production of current collectors by casting methods involves treatment of a melt of lead or lead alloy, which makes the operation troublesome, lowers productivity and causes environmental problems due to the emission of lead vapor. In the production by punching methods, the operation is troublesome and the productivity is low and, moreover, there is a problem of material loss. The production by expand working methods also involves a troublesome operation. Whichever of these production methods may be used, since stress strains remain in the knots which are the intersections of the meshes of the net-like current collector, they are liable to be corroded by the electrolyte. To prevent this, it is necessary to increase the thickness of the current collector, but this approach run counter to reduction of the size or thickness of lead-acid cells using such current collectors. Particularly, this corrosion phenomenon is remarkable in the positive plate. This is because, in the case of the positive plate, as charging and discharging are repeated, the electrode plate expands and stress develops in the current collector, accelerating corrosion phenomenon.
For example, in a cylindrical lead-acid cell, the electricity generating element including the negative and positive plates is in circinately wound form. Such wound generating element is inserted into the cylindrical electrolytic container and after an electrolyte is poured in, the container is sealed to complete the cell.
If the mechanical strength of the net-like current collector used for the electrode plate in such a cylindrical lead-acid cell is too much increased, the electrode plate cannot be circinately wound. Thus, it is inevitable to make the mechanical strength of the current collector somewhat low. Therefore, it becomes necessary to exercise scrupulous care in handling the current collector, while the separator used is also lower in mechanical strength. Therefore, in winding the generating element, special care is necessary and, moreover, it has been very difficult to circinately wind it firmly.